Myalgic encephalomyelitis (ME), also known as benign ME, Royal Free disease, Icelandic disease, epidemic neurasthenia, post-viral syndrome, post-viral fatigue syndrome and by various other names, is a common illness for which there is no known treatment. The characteristic feature of the disease is that a previously healthy person, often a young or middle-aged adult, develops a syndrome characterised by severe fatigue, muscle aching, loss of concentration, headache and palpitations, the last probably indicating an abnormality of cardiac rhythm. The syndrome persists for months or years. The cause is unknown, but a careful history often reveals the occurrence of a febrile viral infection prior to the onset of the syndrome. The majority of patients have some evidence of impaired immune function and of chronic viral infections. Coxsackle and Epstein-Barr viruses are commonly involved but no single specific viral type has been implicated. It seems probable that a variety of viruses can initiate the syndrome in individuals who have an impaired immune response which pre-disposes to chronic infections.